Smog Check

I took my 2000 Nissan Sentra car to the dealers to be smog checked (Calif.) but went on a hike before returning. When I got there the technician said I needed to do a fuel system flush. He also listed other items that totaled about $400. I only did the changing of the transmission fluid ($90) that he said smelled burnt. I asked him if the car passed the smog test and he said probably because he wasn’t told that it didn’t. (The car has about 90,000 miles on it and gets 35 mpg on the highway with the A/C on and going 80 mph and even up mountainous roads.) When I got home he had left a message earlier that I should do a fuel system flush BEFORE I did the smog check to make sure I passed. So I got out my results from two years ago. Two years ago my NO measured 7 PPM at 15 mph and 3 PPM at 25 mph. This test shows 409 at 15 mph and 155 at 25 mph. This is a drastic increase. My question is: Can they futz with the car to make it get a bad reading and make you feel insecure about your car which has been running fantastically? Thank you for your response.

Troubling on how to answer this one. I have never seen an intentional vacuum leak put on a car it but lean mixtures will send the NO up (along with temp and EGR problems)But you do not have a check engine light and you passed your test,I am a bit perplexed what to say. How close did you come to failing due to a NO reading? Can you interpet and post any others? I am sure we forum members can put our minds on this one and come up with a course of action for you.

It just may be a flush too.

Thank you. The maximum NO at 15 mpg is 467 and the maximum NO allowed at 25 mpg is 754. The 15 mpg reading two years ago was 7 and now it is 409. I just feel suspicious because I was told I should get the repair BEFORE the smog test on the phone message, but when I came to the place before hearing the message when I said I didn’t want the repair and then asked if I had passed I was told “probably.” So it just doesn’t jive to me. That seems to be a drastic jump for a car that runs perfectly and gets great gas mileage.

Does anyone think emissions tests do ANYTHING to improve air quality???if so, WHY??

This thread demonstrates EXACTLY what emissions test accomplish…NOTHING.

Sounds to me like a ripoff.
A fuel system flush won’t fix a high NOx problem.

High NOx can be caused by a problem in the EGR system, not related to the fuel system.

By the numbers you posted it should have passed.

I would leave it be and go to a different shop next time it needs to be smog checked.


Frankly that sounds like a wallet flush to me. I doubt if they did anything to your car to cause a problem, but just used the opportunity to sell you additional services.

I hate to say this, but IMO once you get transmission fluid smelling off, it may have already caused problems. Hopefully he was exaggerating that one. Assuming he did change the fluid and did it properly (did he clean the filter?) that was service that was needed.

There were just too many things going on here. I would be a lot more comfortable if you were at an independent local mechanic who does not do smog testing, or was looking at your car after it had passed the test.

The fuel system flush is almost always a wallet flush.

Thanks. I looked at records later and found I had done a transmission flush somewhere else a few years ago. But I did it again. Probably doesn’t hurt. I didn’t have the filter cleaned because my emmision scores had been so good the time two years before and he hadn’t given the results yet. I did pass, but barely with the NO. I’m an environment loving person (we have solar electric panels and make all of our electricity). But I just didn’t trust the sales job he was doing on me. I will go to a smog only station next time. I sure feel lost since our good mechanic, Al Davis of Al’s Auto in Escondido, CA, of 25 years retired and moved away to Tuscon, AZ. He would tell us the things we really needed and not give us a fishing list. He sold his shop to a fellow who wasn’t as good and the new guy went out of business. Sure wish I could find the mechanics he used to use. But thanks again. I am certain to go somewhere else for my smog check next year.

One way the NO readings can be much higher than previous testings is if the catalytic converter is not sufficiently warmed up. If the smog mechanic was hurrying and did not run the car on the rollers long enough before he ran the specified test speeds, the NO can read high because the catalytic converter has not fully lite off.

What I usually do when I have my car smogged is to drive it at freeway speed for about 10 miles and bring it to the shop immediately. I choose a shop that is not too busy and choose a time when the mechanic will probably not have a line of cars. Also I watch the testing procedure to make sure the mechanic does not cut corners.

One incident that happened to me during a smog check was that my gas cap was deemed leaking. When I looked at it the O-ring was missing. When I mentioned that the ring was there when I checked the car the night before, he was able to find an O-ring that allowed the cap to pass. Later I realized that the cap that was tested did not belong to my car because the tightening click was different than I remembered. He had switched caps inadvertently or advertently. I would have had to pay for a retest if I had not caught that. Now I mark my cap with the vehicles license number and check it afterward.

Hope this helps you.

Whether a car runs great, or not, has almost no bearing on how the car’s emissions controls perform. When the emissions get really bad (150% of maximum permitted values), the engine computer will turn on the check engine light. The light indicates that a sensor CIRCUIT isn’t showing the correct values. It does NOT measure emissions, directly.
California, unlike other states, doesn’t want to wait until the exhaust emissions get to greater than allowed levels. That’s why California uses the tailpipe emissions test.
EGR (Exhaust Gas Recirculation) is a solution which was designed to lower NOx (nitrogen oxides) exhaust emissions. It’s so effective that almost all cars use it.
To keep an emission system operating at optimum levels requires a lot of little tweaks, and touches. Unfortunately, some mechanics don’t know how, or when, to perform those tweaks and touches. Word-of-mouth may find you a suitable mechanic.
Your car needs an EGR system flow test, maybe cleaning, and tweaking and touching to bring it to full operability, which will lower NOx…